BRYOLOGICAL INTERACTION-Chapter 4-6

65 CHAPTER 4-6 INVERTEBRATES: MOLLUSKS Figure 1. Slug on a Fissidens species. Photo by Janice Glime. Mollusca – Mollusks Glistening trails of pearly mucous criss-cross mats and also seemed to be a preferred food. Perhaps we need to turfs of green, signalling the passing of snails and slugs on searach at night when the snails and slugs are more active. the low-growing bryophytes (Figure 1). In California, the white desert snail Eremarionta immaculata is more common on lichens and mosses than on other plant detritus and rocks (Wiesenborn 2003). Wiesenborn suggested that the snails might find more food and moisture there. Are these mollusks simply travelling from one place to another across the moist moss surface, or do they have a more dastardly purpose for traversing these miniature forests? Quantitative information on snails and slugs among bryophytes is scarce, and often only mentions that bryophytes are abundant in the habitat (e.g. Nekola 2002), but we might be able to glean some information from a study by Grime and Blythe (1969). In collections totalling 82.4 g of moss, they examined snail populations in a 0.75 m2 plot each morning on 7, 8, 9, & 12 September 1966. The copse snail, Arianta arbustorum (Figure 2), numbered 0, 7, 2, and 6 on those days, respectively, with weights of Figure 2. The copse snail, Arianta arbustorum, in 0.0, 8.5, 2.4, & 7.3 per 100 g dry mass of moss. They were Stockholm, Sweden. Photo by Håkan Svensson through most abundant on the stinging nettle, Urtica dioica, which Wikimedia Commons. 66 Chapter 4-6: Invertebrates: Mollusks In the Pacific Northwest, USA, unusual jumping slugs gastropods Punctum pygmaeum and Pumilla muscorum in the genus Hemphillia (Figure 3) prefer coarse woody among the terrestrial mosses Plagiobryum zieri, Hypnum debris or moss mats on decaying logs (Leonard & Ovaska cupressiforme (Figure 4), and Tortella tortuosa. From my 2003). Their ability to roll or flap violently may break their own observations, it appears that snails and slugs are surface tension, permitting them to move quickly, and common on moss clumps, but finding documentation on giving the appearance that they are jumping. Evidence also their use of moss evades even the aggressive Google suggests that they smear their slime trail, confusing search. potential predators. In Canada, some of these species seem Snails can sometimes occur in significant numbers in safe from extinction due to sufficient abundance, but others moss habitats. Their need for a moist environment (Pratt are endangered due to increasing patchiness of suitable 1935) would seemingly attract snails to the mosses as a habitats. The 1994 NW Forest Plan regulates ground moist substrate. Grime and Blythe (1969) found average disturbance activities on federal lands in northern morning populations of up to 8.5 Arianta arbustorum snails California to Washington, protecting "survey and manage" per 100 g dry weight of moss in early September at species, including several species of jumping slugs. Hence, Winnats Pass in Derbyshire, England. This was surpassed protection of these slugs can help in the protection of only by those on Urtica dioica (stinging nettle) reaching mosses in these areas. However, the Bush administration 14.4 and Mercurialis perennis (dogs mercury) reaching was not sympathetic to this protection and it could be lost 16.2. Nevertheless, it takes a lot of dry moss to make 100 at any time with a change in administrative philosophy. g. However, nighttime activity by many snails is likely to be greater, although little snails may actually seek refuge in mosses during the day. Furthermore, snails like Arianta arbustorum typically climb, often to a considerable height, to obtain food. Bryophytes just don't fit into this behavior pattern, so the behavior of Arianta arbustorum may not reflect that of the small snails. Figure 4. Slug on Hypnum. Photo by Janice Glime. Szlavecz (1986) determined that snail size plays an important role in their behavior. Although one might think that larger animals need to eat more, it seems that larger Monadenia hillebrandi mariposa (see Figure 5) instead spends more time crawling and less time feeding, Figure 3. Hemphillia glandulosa, the warty jumping slug, on permitting it to travel farther. Although it prefers leaf litter, moss. The two photos illustrate the variability in its coloration. Photos by Kristiina Ovaska. it consumes mosses as well (Figure 6). One would expect that size would also constrain movement among the bryophytes and restrict larger snails to the surface. Bryophytes as Home Because of their small movement space, bryophytes Wiesenborn (2003) observed snails in the Riverside can serve as safe sites for smaller snails. Birds can be Mountains of California and found that the active snails significant consumers of snails, particularly during preferred epiphytic mosses and lichens compared to plant migration (Shachak & Steinberger 1980), and bryophytes detritus and four sizes of rocks. They suggested that the can make the snails less conspicuous, if not hiding them epiphytes could provide these snails with food or moisture. completely. In terrestrial habitats, arachnids such as In the sub-Antarctic Marion Island, the slug Deroceros spiders and daddy-long-legs (Opiliones) are also predators caruanae lives in moist bryophyte communities as well as on snails (Nyffeler and Symondson 2001). While some on decaying bryophytes (Smith 1992). In boggy habitats, spiders can probably navigate the spaces within the moss gastropods have little choice but to travel across bryophytes mat, it seems unlikely that mature daddy-long-legs could (Stanisic 1996). In a study of bryophyte inhabitants in the manage without getting caught. In addition to the Bükk Mountains of Hungary, Varga (2008) found the arachnids, Carabid beetles prey on terrestrial gastropods Chapter 4-6: Invertebrates: Mollusks 67 (Symondson 2004). Some of these beetles use a pump In streams, it is likely that snails find mosses as a safe mechanism to extract the gastropod remains from its shell. site from the current. Habdija et al. (2004) rarely found any gastropods on bryophytes at velocities of greater than 70 cm s-1, whereas oligochaetes became more abundant at higher velocities. Flow rates are much slower within the moss mats, thus providing a haven for feeding where the current is unlikely to dislodge the snails and slugs. This also provides them protection from predators such as fish (mostly), ducks, shore birds, and amphibians (Pennak 1953). Figure 5. Monadenia fidelis on mosses. Photo by Walter Siegmund through Wikipedia. Figure 7. Prophysaon vanattae, the scarletback taildropper, can be found hiding under mosses. Photo by Kristiina Ovaska. Figure 8. Prophysaon vanattae with eggs on a moss. Photo by Kristiina Ovaska. Bryophytes as Food Snails and slugs have a rasping tongue that destroys the epidermis of tracheophytes (Grime & Blythe 1969), but what does it do to moss leaves only one cell thick? Apparently it makes them potential food (Szlavecz 1986), enabling these mollusks to consume even the tough capsule (Davidson & Longton 1987, Davidson et al. 1990). Nevertheless, the palatability index for bryophytes is low (Jennings & Barkham 1975). And snails and slugs seem to be less interested in grazing things with awns than those without. Robin Stevenson (pers. comm. January 2008; Figure 9) has seen Bryum argenteum that is completely Figure 6. Laboratory selection of foods by the snail grazed over, but never observed such grazing on an awned Monadenia hillebrandi mariposa. Upper: all data combined. Grimmia species. Could it just be that there is no nutrition Lower: juveniles vs adults. Redrawn from Szlavecz 1986. in an awn, or do they have trouble gliding across the furry tips of leaves? The slug Prophysaon vanattae (scarletback But awns may not deter all snails. Szlavecz (1986) taildropper; Figure 7) is one of those snails that seems to was able to identify the awned Grimmia trichophylla in the find a safe site under mosses on trees on Vancouver Island, feces of the California snail, Monadenia hillebrandi Canada (Kristiina Ovaska, pers. comm. 30 June 2009). But mariposa and also demonstrated that the spine tips of the it also hangs on epiphytic moss mats in the moist deciduous tracheophyte Selaginella hanseni did not deter feeding or forest there and may even lay eggs there (Figure 8). crawling. Michael Lüth has observed snails grazing on 68 Chapter 4-6: Invertebrates: Mollusks Orthotrichum and Terry McIntosh has seen slugs grazing on other bryophytes, both indicating that the damage to the moss was similar to that shown in Figure 10 (Bryonet 12 January 2008). On the other hand, Frank Greven (Bryonet 13 January 2008) has seen this pattern as a result of grazing by isopods (wood lice). Robin Stevenson (pers. comm. 14 January 2008) agrees that isopods might be deterred by the awns, causing them to eat in such a pattern. But why would slugs or snails climb up the bridge coping to the moss for such a limited nutrient source? Figure 11. Shell of Limicolaria aurora. Photo by David G. Robinson, USDA APHIS PPQ at Bugwood.org. Algae growing on mosses, especially in the aquatic habitat, could be a prominent source of food. In the Negev Desert, adult desert snails (Sphincterochila zonata) fed exclusively on algae on the soil surface, creating an algal turnover of 142 kg hectare-1, despite being active for only 8-27 days in winter during the rainy period (Shachak & Steinberger 1980). Other Negev Desert snails feed on the mosses themselves. Trochoidea seetzeni feeds on shrubs there, but its feces indicate that it also feeds on the moss Desmatodon convolutus (Yom-Tov & Galun 1971). Clearly for some slugs and snails there are bryophytes Figure 9. Bryum argenteum, a moss with no awns and a food that do indeed seem palatable. Ochi (1960) reported that source for snails and slugs.

Load more